This invention relates to a moistureproof structure for module circuits. More particularly, this invention relates to a moistureproof structure for module circuits that are to be used at high frequencies not lower than a few GHz.
A class of module circuits that are used at high frequencies not lower than a few GHz are conventionally composed of coaxial lines or triplet lines so that the circuits are protected against wire corrosion and variations in electrical characteristics due to moisture and vapor condensation. Another class of module circuits having striplines formed on a substrate to provide electronic circuits are either sealed in ceramic containers or provided with a moistureproof coat as described in Japanese Patent Public Disclosure No. 17492/1989, whereby the circuits are protected against wire corrosion and variations in electrical characteristics.
However, the moistureproof structures that use coaxial lines require cumbersome connecting operations and those which use triplet lines take much time to form the desired circuit. Hence, either type of structure suffers from the disadvantage of increased production cost.
The moistureproof structures that use striplines are free from the problem associated with wire connecting and circuit forming operation. However, if ceramic containers are used, problems arise such as the high cost of ceramic containers and unsuitability for high-volume production.
If moistureproof coats are applied, the electrical characteristics of the circuit will vary greatly since the dielectric constant of the coating material is substantially higher than that of air. It is theoretically possible to design the circuit allowing for the possible variations in electrical characteristics but, in practice, the electrical characteristics of the circuit will vary with the thickness of the moistureproof coat, and it is necessary to insure that the circuit is always coated with the correct thickness of the coat that complies with the circuit design. However, controlling thicknesses of less than a hundred microns within the limits of .+-. a few microns is very difficult to accomplish. In addition, the condensation of water vapor on the moistureproof coat over striplines is particularly undesirable since water having a relative dielectric constant as large as about 80 will have large effects on the electrical characteristics of the circuit. In order to avoid the adverse effect of vapor condensation by means of the moistureproof coat, it must have a thickness of at least 1 mm; however, if a moistureproof coating material which has a high dielectric constant is formed at a thickness of 1 mm and more, the coating material alone is sufficient to cause variations in the electrical characteristics of the circuit that are too great to be effectively absorbed by circuit design.